The present invention provides superior means for forming uniformly thin coatings on substrates such as cylinders. More particularly it provides a distributor device which permits obtaining such uniformity, along with a coating system which includes an apparatus and a method.
It is difficult to provide a thin coating of uniform thickness from a thixotropic material, and particularly difficult if the material is a plastisol such as polyvinyl fluoride in a latent solvent and if the substrate to be coated has a curved or cylindrical shape. Spray application tends to give an irregular or wavey orange peel texture. Dipping is not ideal because of the need for sheer forces to make thixotropic material flow. The use of thixotropic material rather than material with Newtonian viscosity behavior is important in avoiding sagging and curtaining of coatings.
For certain applications, close control of dielectric and other physical properties depend on uniformly thin coatings. While conventional coating techniques may lead to thickness variations of +/-10 or 15 to 20% with thixotropic materials, it is desireable to be able to obtain variations of +/-5%, 21/2%, or even less.
Polyvinyl fluoride (PVF) is generally not soluble at room temperature in conventional solvents, however, it can be put into solution with so called latent solvents. A dispersion of PVF powder is suspended in latent solvent and heated to a first temperature at which a gel is formed and then to a higher second temperature at which a solution is formed. Latent solvents and other technology useful in handling PVF are discussed in U.S. Pat. Nos. 2,953,818--Bartron (Sept. 27, 1980) and 3,139,470--Prengle et al. (June 30, 1964), both incorporated herein by reference.
Technology for applying coatings to cylindrical surfaces, including plastisols such as polyvinyl chloride (PVC) in latent solvents, and other plastic coatings is described in U.S. Pat. Nos.:
2,647,296--Shive (Aug. 4, 1953), PA1 2,647,488--Shive (Aug. 4, 1953), PA1 3,690,294--Harper (Sept. 12, 1972), PA1 3,896,764--Kindl, et al. (July 29, 1975), and PA1 4,212,908--Hendy et al. (July 15, 1980), as well as U.S.S.R. Pat. No. PA1 910,444--as U.S.S.R. Urals (Mar. 3, 1982).
The two Shive patents involve simply passing a wire through a bath of coating material and wiping the coated wire on its exit from the bath. Harper has a distributor head revolving around the substrate to be coated with interior paddles which aid in distributing the coating onto the substrate. The Soviet patent has a series of ports arranged longitudinally along the cylindrical substrate or pipe to give constant pressure along the length of the pipe.
Techniques of the prior art are not capable of giving results as good as desired for certain applications.